by HUMBERTO BASILIO Ed. ANDREA THOMPSON
Lime granules trapped in ancient walls show Romans relied on a reactive hot-mix method to making concrete that could now inspire modern engineers
Ancient Romans built arched bridges, waterproof port infrastructure and aqueducts that enabled the rise of their empire and that are still standing—and often still used. They did so with a type of cement that is far sturdier than what is used today, but exactly how Roman cement was made was something of a mystery. Now researchers have found proof of an explanation they had proposed in 2023 that could offer insights into how to build longer-lasting concrete today.
In his first-century B.C.E. work De Architectura, Vitruvius, one of the most famous architects of the Roman Empire, described Roman cement as being made with what we today call slaked lime, or hydrated, heated limestone. But based on the discovery of the makeup of chunks called “lime clasts” found at a previous excavation in Pompeii, Massachusetts Institute of Technology environmental engineer Admir Masic and his colleagues proposed in a 2023 paper that ancient builders instead used a process called “hot mixing.” In this method, highly reactive quicklime (dry heated limestone) is mixed with volcanic ash and water, setting off a chemical reaction that produces heat and gives the material self-healing capabilities.
To reaffirm his discovery, Masic and his team returned to Pompeii in 2024 and visited a house that was under renovation when Mount Vesuvius erupted, freezing the place in time. “I literally felt like I was a worker in 79 C.E.,” Masic says.
Inside one of the rooms, among stones, roof tiles and tools, the researchers found large piles of dry, premixed mortar ingredients—a blend of volcanic ash and granules of quicklime—waiting to be hydrated and applied to walls, Masic says.
Scientific American for more